The present invention relates to a communication system in which communication is carried out through a communication medium such as a coaxial cable, an optical cable, a telephone circuit or satellite broadcasting and an apparatus for recording and reproducing a signal communicated through the communication medium and more particularly, to a communication system for encrypted data and an apparatus for selectively recording and reproducing a received analog signal and a received digital signal.
A technique concerning a system for encryption and decryption of a video signal and an audio signal as well as fee charging is described in, for example, "Transactions The Institute of Television Engineers of Japan", Vol. 46, No. 1, January, 1992, pp. 31-39.
The above paper describes a technique concerning the system for encryption and decryption of a video signal and an audio signal as well as fee charging in satellite communication service. However, that paper fails to disclose method and technique which can store a received signal while protecting a copyright of the signal and can charge fees of decryption and storage of the signal.
A technique of selecting a digital signal or an analog signal and recording it on the same magnetic recording and reproducing apparatus is described in, for example, JP-A-5-207507.
Available for a domestic magnetic recording and reproducing apparatus (hereinafter referred to as VTR) are, for example, VHS, .beta. and 8 millimeter-VTR standards. In any of these standards, an analog television signal is FM modulated and then recorded. It is stipulated that an audio signal can also be FM modulated and recorded in accordance with each standard. An apparatus for multiplexing, recording and reproducing analog video and audio signals in this manner is disclosed in U.S Pat. No. 4,591,924.
When a television signal is FM modulated for analog recording, there arises a problem that the S/N ratio is inevitably deteriorated owing to recording and reproduction. To cope with this problem, the television signal is digitized and then recorded. For example, D-1, D-2 and D-3 standards prescribed by SMPTE are available. These standards are determined by taking the use for broadcasting into account and therefore, a tape cassette different from that used for the aforementioned domestic VTR is used under these standards.
The fact that quality of recording and reproduction is improved by recording and reproducing a digital signal is desirable but when user's merit is taken into consideration, it is preferable that the conventional analog signal be also recorded and reproduced. In other words, exchangeable reproduction of analog and digital signals must be taken into account.
This requires that at least the shape of a cassette to be used be the same for analog and digital signals and the tape transport system be also the same for analog and digital signals. Further, by making the recording and reproducing head or at least the azimuth angle of the head identical for analog and digital signals, exchangeability can be facilitated and cost reduction can be ensured.
But it has been found that the above expedient faces a new problem. For example, when the digitally recorded cassette is inserted by mistake into the conventional analog VTR, the recorded digital signal is mistaken for an FM signal and is FM demodulated inadvertently. Especially when the FM demodulated signal is an audio signal, the demodulated audio signal has a large amplitude and so large a sound that possibly surprises persons involved and damages the apparatus is generated from a loudspeaker.
The aforementioned conventional technique fails to show what problem arises when the digitally recorded cassette is loaded on the conventional analog VTR. Needless to say, the conventional technique does not clarify how to solve the problem.